1. Field of the Invention
The present invention relates to ink jet apparatus. In particular, the invention relates to an ink jet apparatus for ejecting ink to form an image on a recording medium, and an ink jet recorder including the ink jet apparatus.
2. Description of Related Art
Recently, the market for non-impact printers has enlarged greatly in place of the market for impact printers. Among the non-impact printers, ink jet printers may have the simplest principles and be easiest of multiple gradation and colorization. Ink jet printers of the drop-on-demand type eject the only ink for printing. Ink jet printers of this type are coming rapidly into wide use because of high ejection efficiency and low running costs.
For example, U.S. Pat. Nos. 5,028,936, 5,003,679, 4,992,808, 4,887,100 and 4,879,568 disclose ink jet apparatus of the shear mode type for use in a drop-on-demand type ink jet printer. Piezoelectric material is used for the disclosed apparatus. FIGS. 24A and 24B and FIG. 25 of the accompanying drawings show a conventional ink jet apparatus of this type, which comprises a print head 600. The head 600 includes a base wall 601 and a top wall 602 between which extend shear mode actuator walls 603a-603h. The actuator walls 603a-603h have upper parts 605 and lower parts 607 made of piezoelectric material. The wall parts 605 and 607 are bonded to the walls 602 and 601, respectively, and polarized in the opposite directions of arrows 609 and 611, respectively. The actuator walls 603a, 603c, 603e and 603g pair with the actuator walls 603b, 603d, 603f and 603h, respectively, to define an ink channel 613 between each pair of actuator walls. The actuator walls 603b, 603d and 603f pair with the actuator walls 603c, 603e and 603g, respectively, to define an air space 615 between each pair of actuator walls. The spaces 615 are narrower than the channels 613.
At one end of the channels 613 is secured a nozzle plate 617 formed with nozzles 618 each for one of the channels. The other ends of the channels 613 are connected through a manifold 626 to an ink cartridge or another ink supply (not shown). The manifold 626 includes a front wall 627 and a rear wall 628. The front wall 627 is formed with holes each communicating with one of the channels 613. The rear wall 628 closes the space in the rear of the front wall 627 between the rear ends of the base wall 601 and top wall 602. Ink can be supplied from the supply to the space between the front wall 627 and rear wall 628, and then be distributed to the channels 613.
The longer four sides of each channel 613 are lined with an electrode 619. The longer four sides of each space 615 are lined with an electrode 621. The outer sides of the actuator walls 603a and 603h are each lined with an electrode 621. The electrodes 619 and 621 take the form of metallized layers. The electrode 619 around each channel 613 is passivated with an insulating layer (not shown) for insulation from ink. The electrodes 619 in the channels 613 are connected to a drive circuit 640. Under the control of a control circuit 641, the drive circuit 640 can generate a voltage and apply it to these electrodes. The other electrodes 621 are connected to a ground return 623.
In operation, the voltage applied to the electrode 619 in each channel 613 causes the actuator walls facing the channel to deform piezoelectrically in such directions that the channel enlarges in volume. If, as shown in FIG. 25, a predetermined voltage of E volts is applied to the electrode 619 between the actuator walls 603e and 603f, for instance, electric fields are generated in these walls in the opposite directions of arrows 631 and 632. This deforms the walls 603e and 603f piezoelectrically in such directions that the associated channel 613 enlarges, reducing the pressure in this channel to a negative pressure.
The voltage applied to the electrode 619 is held for a period L/V where L is the channel length and V is the sound velocity in the ink in the channel 613. While the voltage is applied, ink is supplied from the supply to the channel 613. The period L/V is the one-way propagation delay time T which it takes for the pressure wave in the channel 613 to be propagated one way longitudinally of the channel.
According to the theory of pressure wave propagation, the negative pressure in the channel 613 reverses into a positive pressure when the period L/V passes after the voltage is applied to the electrode 619. When the period L/V passes after the voltage is applied to the electrode 619, the voltage is returned to zero volt. This allows the deformed actuator walls 603e and 603f to return to their original condition (FIGS. 24A and 24B), generating a positive pressure in the channel 613. This pressure is added to the pressure reversed to be positive. As a result, a relatively high pressure develops in that portion of the channel 613 which is near to the associated nozzle 618, ejecting ink out through the nozzle. The ejected ink sticks to a surface of printing paper or another recording medium to form an image on it.
As stated above, pressure wave vibration of ink is generated in the channel 613 to eject ink out through the nozzle 618. The applicant has devised, in U.S. patent application Ser. No. 09/007,756, substantial cancellation of the residual pressure wave vibration of ink in the channel 613 after the ejection. The cancellation involves increasing and decreasing the volume of the channel 613 by applying the voltage of E volts to the electrode 619 again at a predetermined time and subsequently returning the voltage to 0 volt. The cancellation damps the residual pressure wave vibration in the channel 613 quickly and early. This prevents ink from being ejected or dropped accidentally through the nozzle 618 by the residual vibration. Besides, this enables early transition to the process in accordance with the next print command. It is therefore possible to form a more faithful image on the recording medium, and improve the print speed.
After ink is ejected out through the nozzle 618 in a predetermined cycle, there may or may not be a print command for the next cycle. The applicant has also devised in the above-identified U.S. patent application, cancelling the residual pressure wave vibration if there is no print command for the next cycle, and carrying out no such cancellation if there is a print command for this cycle.
In other words, if there is no print command for the cycle following a predetermined cycle, an accidental drop of ink may occur, and therefore the residual pressure wave vibration should be cancelled. This results in better image formation not stained or spotted by scattered ink.
If there is a print command for the next cycle, the residual pressure wave vibration in the channel 613 should be utilized positively. Specifically, this vibration should be added to the pressure wave vibration generated in accordance with the command for this cycle. This generates greater pressure wave vibration for ejection of a larger ink droplet through the nozzle 618. Larger ink droplets increase the print density to form a thicker and clearer image.
In order to switch between the execution and no execution of such cancellation depending on whether there is a print command for the cycle following a predetermined cycle, it is necessary to accurately control the voltage application from the drive circuit 640 to the electrode 619. This requires the control circuit 641 to accurately control the drive circuit 640.